This invention relates to an antenna assembly which enables a radio to operate with either an existing antenna, or with an external antenna. The invention may be applied, for example, to a portable radio and, in particular, to a radio telephone.
A radio intended for two-way communication usually operates with either an external fixed rod or adjustable (e.g. telescopic) antenna, or with an internal antenna. In either case, such antennas will be included within the term `existing antenna` as used herein.
Some known portable radio systems have two existing antennas, i.e. an internal element together with a retractable element, and are also equipped with means for automatically switching between the two elements according to the physical position of the retractable element. For example, the internal element may be some form of parallel plate antenna and the retractable element may be of the telescopic tube type. Such a system generally has the disadvantage that no immediate means is available to connect an external independent antenna in place of the self-contained system.
British patent GB-A-844,968 discloses an arrangement for connecting an external antenna to a radio receiver having a single existing, internal antenna intended for use in a vehicle. A housing is provided into which the radio is slidably mounted. A jack plug connected to the external antenna is fixedly mounted on the rear wall of the housing so that when the radio is fully inserted into the housing the jack plug mates with an antenna socket provided in the back of the radio apparatus. The housing acts to screen the internal antenna and thereby render it inoperative when the jack plug and hence the external antenna are connected. This arrangement requires the radio to be almost completely encased within the housing, so making the radio relatively inaccessible when connected to the external antenna. Also, since the internal antenna remains physically and electrically connected to the radio circuits, this arrangement is not suitable for a radio intended for transmission as well as reception.